The reason why NetherRealm is working on Mortal Kombat 12 over Injustice 3 is due to the WarnerMedia and Discovery, Inc. merger earlier this year.
According to several reports, the merged company will split up Warner Bros Games sometime in the future.
This suggests that NetherRealm is erring on the side of caution if the leaks are true.
On the off-chance that NetherRealm was working on Injustice 3 and lost the license to Batman and others, it would mean that they'd have to scrap development on the game midway.
NetherRealm's decision to pivot their resources towards a safer project does make sense on this basis.
It just doesn't line up with the earlier reports that NetherRealm's next project is a Marvel fighting game.